Somalia's al-Shabab militants are now using Twitter. You can follow the account @HSMPress – derived from the Shabab's full name, Harakat al-Shabab al-Mujahideen, or Movement of Freedom Fighter Youth – for pithy updates on their violent campaign to bring a rigorous version of conservative Islam to east Africa and beyond, all in 140 characters or less. The account already has more than 3,000 followers. Are the al-Shabab tweeters jealous of their Afghan Taliban counterparts whose own account @alemarahweb has 6,000 followers? Like all Twitter users they'd deny it, of course – and probably be lying.

It is not just Twitter of course. And the increased use of social media by Islamic militants has provoked two broad responses. The first is surprise. The second is that Twitter, Facebook, YouTube et al will prove an enormous boost to such outfits. Neither response is justified.

The apparent shock at the use of social media is odd. Though often described as "medieval", militant groups are actually extremely modern, with a worldview built from a mixture of very contemporary religious and secular sources. Their project is not to reject modernity as such but to impose an alternative version. It is the use of technology that is a potential problem, not its existence. Like television, Twitter might be a corrupting influence in the hands of some – but not in the hands of al-Shabab themselves.

There is also the obvious point that militants do not come from Mars. Back in 2003, senior British officials said the danger in the UK was from militants "hiding as ordinary people". They then worked out, after the 7/7 bombings, that militants were ordinary people. This is still the case. So there is no reason that they – particularly as they are usually in the 18-30 age group – should differ from the population at large.

As for the worry about the new power of social media in the hands of militants, we should probably relax. Al-Shabab's tweeter is witty, sharp and articulate, and undoubtedly attractive to the odd aspirant jihadi. There are many, many examples of social media playing a role in radicalisation in recent years. The late Anwar al'Awlaki, an anglophone propagandist based in the Yemen, became a one-man central node for an internet-based network around the world. For the so-called "lone wolves" (and some who are less solitary), there is no doubt that social media can play a significant role.

But its importance is overblown. Militancy involves a complex web of personal associations and the strongest influences are brothers, fathers and friends, not virtual web-based communities. Mark Sageman, a psychologist, sociologist and CIA veteran, pointed out to a court in the United States that the largest number of recruits to al-Qaida came in the 1990s, before widespread use of the internet.

At the height of the Iraq war, MI5 told me that it was what was on the News at Ten that radicalised people, not specialised extremist websites. Most significantly, Twitter will never be a substitute for grassroots activism. In much of the Islamic world, social media is only for super-connected local elites or supporters in far off countries. Neither are much use on the ground, where it counts. Social media can bring in donations or some foreign recruits. It can aid communication with some logistics and facilitate propaganda operations, but it is not much use in a firefight with Saudi, Iraqi or Pakistani security forces. Twitter won't help al-Shabab retake Mogadishu or the Taliban reach Kabul in any meaningful way.

Social media gets huge attention in the western media, mirroring our own preoccupations and lifestyles. The western focus on the twitterati in Egypt and elsewhere has meant that not only are we surprised that Islamic militants tweet too, but we are shocked to find that there are still very considerable numbers of people who are not extremists but are deeply conservative and far from westernised. They do not tweet, but, as we are seeing in Egypt, they do vote.

Terrorism is a social activity like any other. Militant or moderate, you can spend as much time as you like online, but eventually you have to come back to the real world.